diff --git a/ffmpeg.c b/ffmpeg.c
index cdc78ea5d2..d12af7b232 100644
--- a/ffmpeg.c
+++ b/ffmpeg.c
@@ -3235,7 +3235,7 @@ static void prepare_grab(void)
     if (has_video) {
         AVInputFormat *fmt1;
 #warning FIXME: find a better interface
-        if(!strncmp(video_device,"x11:",4)) {
+        if(video_device&&!strncmp(video_device,"x11:",4)) {
             video_grab_format="x11grab";
         }
         fmt1 = av_find_input_format(video_grab_format);